Snap-action type electric switches have found considerable success for a long period of time in applications where power load levels are substantial. Most of these snap-action switches employ a movable contact element that traverses between two stationary contacts, and a coiled spring that biases the movable contact toward each of the stationary contacts depending upon which side of a central position the movable contact is in. This over-center spring geometry moves the movable contact very rapidly toward and away from the stationary contacts, minimizing the possibility of contact dithering and arcing, as well as contact welding.
Such switches inherently have considerable hysteresis, i.e. switch contacts will "make" at a given point in the travel of the switch actuator and "break" at a different position of the actuator. If such a switch is operated by a pressure-responsive diaphragm, this make-and-break differential may, for example, cause the switch to actuate at 7 psi but not deactuate until pressure falls to 6 psi. This minimizes dithering of the switch contacts. Such a pressure operated switch is illustrated and described in the Mayer U.S. Pat. No. 4,272,660, assigned to the assignee of the present invention.
The make-and-break pressure settings for diaphragm-operated pressure switches can be adjusted in a variety of ways, such as varying diaphragm geometry, varying the length of the actuating elements in the switch, as well as varying diaphram vs. switch position. However, as far as Applicant is aware, these variations are usually made by the manufacturer of the switch assembly and are not intended to nor may they be readily varied by the ultimate user of the switch.
In some applications, however, it is desirable that the ultimate user of the switch be permitted to vary the switch operating pressure. One such application is in the control of heating units for swimming pool and spa water heating systems. Such systems include electric heating elements across which water conduits pass in and out of contact heat exchange relation. To prevent damage to the heating elements it is desirable that they be shut off when water flow through the heat exchange conduits falls below a predetermined level. Since the rate of water flow varies from one system to another and is in fact variable in many individual systems, it is desirable to afford the ultimate user and the installer and opportunity to adjust the pressure setting at which the heating unit is de- energized.
It is also desirable that the switch assembly be sealed in this environment from the corrosive and other deleterious effects of the swimming pool environment.
However, to date no pressure switch assembly has been provided that is readily adjustable after installation and which is sealed from its environs.
It is therefore a primary object of the present invention to ameliorate the problems noted above in pressure operated switch assemblies.